| Abstract
| - Sialic acid is an essential sugar in biology that plays key roles in numerous cellular processesand interactions. The biosynthesis of sialylated glycoconjugates is catalyzed by five distinct families ofsialyltransferases. In the last 25 years, there has been much research on the enzymes themselves, theirgenes, and their reaction products, but we still do not know the precise molecular mechanism of actionfor this class of glycosyltransferase. We previously reported the first detailed structural and kineticcharacterization of Cst-II, a bifunctional sialyltransferase (CAZy GT-42) from the bacterium Campylobacterjejuni [Chiu et al. (2004) Nat. Struct. Mol. Biol.11, 163−170]. This enzyme can use both Gal-β-1,3/4-Rand Neu5Ac-α-2,3-Gal-β-1,3/4-R as acceptor sugars. A second sialyltransferase from this bacterium, Cst-I, has been shown to utilize solely Gal-β-1,3/4-R as the acceptor sugar in its transferase reaction. Wereport here the structural and kinetic characterization of this monofunctional enzyme, which belongs tothe same sialyltransferase family as Cst-II, in both apo and substrate bound form. Our structural datashow that Cst-I adopts a similar GTA-type glycosyltransferase fold to that of the bifunctional Cst-II, withconservation of several key noncharged catalytic residues. Significant differences are found, however,between the two enzymes in the lid domain region, which is critical to the creation of the acceptor sugarbinding site. Furthermore, molecular modeling of various acceptor sugars within the active sites of theseenzymes provides significant new insights into the structural basis for substrate specificities within thisbiologically important enzyme class.
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